On the scaling properties of the total ${\gamma }^{\ast }\mathrm{p}$ cross section

Abstract

We perform a detailed analysis on the scaling properties of the total ${\gamma }^{\ast }\mathrm{p}$ cross section, ${\sigma }_{{\gamma }^{\ast }\mathrm{p}}$. We write the cross section as a product of two functions $W$ and $V$ representing, respectively, the dynamical degrees of freedom and the contribution from the valence partons. Analyzing data from HERA and fixed target experiments we find that $V$ is nearly independent of $Q^2$ and concentrated at large $x$, while $W$ carries all the information on the $Q^2$ evolution of ${\gamma }^{\ast }\mathrm{p}$. We define the reduced cross section ${\tilde{\sigma }}_{{\gamma }^{\ast }\mathrm{p}}\equiv W={\sigma }_{{\gamma }^{\ast }\mathrm{p}}/V$, and show that it is very close to a generalized homogeneous function. This property gives rise to geometric scaling not only for small $x$, but for all the current measured kinematic plane. As a consequence of our {\em Ansatz} we also obtain a compact parameterization of ${\sigma }_{{\gamma }^{\ast }\mathrm{p}}$ describing all data above $Q^2=1$ GeV${}^2$.

Comments: Major rewriting of the text. Results unchanged, conclusions expanded